1. Field of the Invention
This invention generally relates to video signal processing and, more particularly, to a system and method for losslessly coding video information.
2. Description of the Related Art
Lossless video coding is important for applications such as medical imaging, satellite imaging, digital cinema, where there are technical or legal reasons to prevent any loss of information. However, so far, most video coding standards, such as H.261, H.263, H.263+, MPEG-1, MPEG-2, and MPEG-4, have not addressed this need.
H.26L, also known as MPEG-4 Part 10, Advanced Video Coding (AVC) or ITU-T H.264, is an up-coming video coding standard being developed through the Joint Video Team (JVT) of ITU-T VCEG (video Coding Expert Group) and ISO/IEC MPEG. H.26L has demonstrated much higher compression ratio than existing standards, such as H.263+ and MPEG-4 Advanced Simple Profile (ASP).
FIG. 1 is a schematic block diagram illustrating the basic architecture of an H.26L encoder framework (prior art). The framework inherited the popular hybrid DPCM/transform approach, which has been adopted in many earlier video-coding standards. However, like most existing video coding standard, at this point H.26L does not support lossless video coding, which is enabled by other image coding standards, such as Motion JPEG 2000.
As noted by Ian Richardson (H.264/MPEG-4 Part 10 White Paper, www.vcodex.com), broadcast television and home entertainment have been revolutionized by the advent of digital TV and DVD-video. These applications, and many more, are made possible by the standardization of video compression technology. The next standard in the MPEG series, MPEG4, is enabling a new generation of Internet-based video applications whilst the ITU-T H.263 standard for video compression is now widely used in videoconferencing systems.
MPEG4 (Visual) and H.263 are standards that are based on video compression (“video coding”) technology from circa. 1995. The groups responsible for these standards, the Motion Picture Experts Group (MPEG) and the Video Coding Experts Group (VCEG) are in the final stages of developing a new standard that promises to significantly outperform MPEG4 and H.263, providing better compression of video images together with a range of features supporting high-quality, low-bitrate streaming video. The history of the new standard, “Advanced Video Coding” (AVC), goes back at least 7 years.
After finalizing the original H.263 standard for videotelephony in 1995, the ITU-T Video Coding Experts Group staffed work on two further development areas: a “short-term” effort to add extra features to H.263 (resulting in Version 2 of the standard) and a “long-term”effort to develop a new standard for low bitrate visual communications. The long-term effort led to the draft “H.26L” standard, offering significantly better video compression efficiency than previous ITU-T standards. In 2001, the ISO Motion Picture Experts Group recognized the potential benefits of H.26L and the Joint Video Team (JVT) was formed, including experts from MPEG and VCEG. JVT's main task is to develop the draft H.26L “model” into a full International Standard. In fact, the outcome will be two identical standards: ISO MPEG4 Part 10 of MPEG4 and ITU-T H.264. The “official” title of the new standard is Advanced Video Coding (AVC). However, it is widely known by its old working title, H.26L and by its ITU document number, H.264.
In common with earlier standards (such as MPEG1, MPEG2 and MPEG4), the H.264 draft standard does not explicitly define a CODEC (enCOder/DECoder pair). Rather, the standard defines the syntax of an encoded video bitstream, referred to herein an entropy encoded macroblocks, together with the method of decoding this bitstream. In practice, however, a compliant encoder and decoder are likely to include the functional elements shown in FIG. 1. Whilst the functions shown are likely to be necessary for compliance, there is scope for considerable variation in the structure of the CODEC. The basic functional elements (prediction, transform, quantization, entropy encoding) are little different from previous standards (MPEG1, MPEG2, MPEG4, 11.261, H.263). The important changes in H.264 occur in the details of each functional element.
The encoder includes two dataflow paths, a “forward” path and a “reconstruction” path. An input frame or macroblock, is presented for encoding. The frame is processed in units of a macroblock (MB), corresponding to 16×16 pixels in the original image. Each macroblock is encoded in intra or inter-mode. In either ease, a prediction macroblock is formed based on a reconstructed frame. In intra-mode, the prediction MB is formed from samples in the current frame that have previously encoded, decoded and reconstructed. In inter-mode, the prediction MB is formed by motion-compensated prediction from one or more reference frame(s). The reference frame is the previous encoded frame. However, the prediction for each macroblock may be formed from one or two past or future frames (in time order) that have already been encoded and reconstructed.
The prediction MB is subtracted from the current macroblock to produce a residual or difference macroblock. This is transformed (using a block transform) and quantized to give X, a set of quantized transform coefficients. These coefficients are re-ordered and entropy encoded. The entropy-encoded coefficients, together with side information required to decode the macroblock (such as the macroblock prediction mode, quantizer step size, motion vector information describing how the macroblock was motion-compensated, etc) form the compressed, or encoded bitstream. This is passed to a Network Abstraction Layer (NAL) for transmission or storage.
The quantized macroblock coefficients X are decoded in order to reconstruct a frame for encoding of further macroblocks. The coefficients X are re-scaled and inverse transformed to produce a difference macroblock. This is not identical to the original difference macroblock as the quantization process introduces losses. Therefore, the recovered MB is a distorted version of the original MB.
It would be advantageous if a lossless H.26L or MPEG video encoding process existed.